The present invention relates to a phase detecting circuit for producing a digital phase signal varying with a frequency variation or a phase variation of a two-phase or polyphase AC signal.
In a thyristor phase control type power converter, it is necessary to properly control a firing phase of the thyristor in accordance with the phase of power source voltage. In order to effect such control using digital IC technique, it is required to digitally detect a phase of an AC signal for energizing the thyristor. In case where it is expected that the frequency of an output signal greatly varies as in the case of the dynamotor, a rapid phase control is further required.
A phase detecting circuit including a phase locked loop (PLL) has generally been used for such a digital phase detection. FIG. 1 shows a phase detection circuit for digitally detecting a phase of a three-phase AC signal widely used in the thyristor phase control type power converter.
In the phase detecting circuit shown in FIG. 1, three-phase AC signals S1U, S1V and S1W of 50 Hz are applied to wave shapers 2, 4 and 6 where these signals are converted into rectangular wave signals and then applied to first terminals of comparators 8, 10 and 12. As will be described later, those comparators 8, 10 and 12 receive at the second terminals rectangular wave signals with phases different from one another by 120.degree. and produces rectangular wave signals of 100 Hz including DC components proportional to phase differences between the input signals received at the first and second terminals. The output terminals produced from the comparators 8, 10 and 12 are summed in an adder 14 and then the adder 14 produces an output signal of 300 Hz. The output signal from the adder 14 is applied to a low-pass filter 18 where it is converted into a DC signal proportional to a phase difference between the input signals applied to the first and second input terminals of each comparator 8, 10 and 12. The DC signal from the low-pass filter 18 is supplied to a voltage-frequency converter 16. The voltage-frequency converter 16 produces pulses at a rate corresponding to the input DC signal. The output pulse from the voltage frequency converter 16 is frequency-divided into a signal of 300 Hz by a counter circuit 20 and the 300 Hz signal is then applied to a 6-scale counter circuit 22. A decoder 24 decodes the contents of the counter circuit 22 and supplies rectangular wave signals S2U, S2V and S2W of about 50 Hz with phases different from one another by 120.degree. which are in synchronism with the output pulse from the converter 16 to the second input terminals of the comparators 8, 10 and 12. The contents of the counter circuits 20 and 22 are used to form phase data .theta.1 representing a rectangular wave signal generated from the decoder 24.
In the phase detecting circuit shown in FIG. 1, so long as the three-phase AC signals S1U, S1V and S1W are respectively synichronized with the three-phase rectangular wave signals S2U, S2V and S2W, the converter 16 produces pulses with given frequencies (equal to the frequency division ratio of the counter 20.times.6.times.50 Hz) so that the decoder 24 may continuously produce the output signals S2U, S2V and S2W in phase with the three-phase AC signals S1U, S1V and S1W. Under this condition, if the phase of the three-phase AC signal is brought in advance of that of the three-phase rectangular wave signal from the decoder 24, the comparators 8, 10 and 12 produce output signals containing larger DC components and the V-F converter 16 produces an output pulse with a higher frequency. As a result, the counter 22 operates at a higher speed and the decoder 24 produces three-phase rectangular wave signals S2U, S2V and S2W phase-advanced. In this way, three-phase rectangular wave signals S2U, S2V and S2W are controlled to be substantially always in phase with the three-phase AC signals S1U, S1V and S1W and phase data .theta.1 representing the phase of the three-phase rectangular wave signals S2U, S2V and S2W is always taken out from the counters 20 and 22.
In the circuit shown in FIG. 1, when the low-pass filter 18 is formed to have proportion and integration function or PI function, it is possible to change the phases of the three-phase rectangular wave signals following phase changes of the three-phase AC signals S1U, S1V and S1W without significant delay.
The phase detecting circuit shown in FIG. 1 has the following disadvantages. The first is that the constructions of wave shapers 2, 4 and 6 are complicated. The input three-phase AC signals S1U, S1V and S1W are sine wave signals including a noise component and distortion component. Accordingly, when such sine wave signals are wave-shaped, those signals are susceptible to the noise and the distortion. In order to remove the influence of the noise and the distortion, the three-phase AC signal is filtered out and then is wave-shaped. Since the phase of the three-phase AC signal fed through the filter is delayed and particularly an amount of the phase delay changes according to a frequency change of the three-phase AC signal, the phase detecting signal will contain a phase difference corresponding to the frequency change, even if the PLL properly operates. The compensation for the phase difference needs a complex circuit for detecting the frequency of the three-phase AC signal to control the amount of phase delay corresponding to the frequency change.
Since the comparators 8, 10 and 12 compares phases of the rectangular wave signals, the phase comparison is performed for each 180.degree. phase angle. Accordingly, even when three comparators are used, the phase comparison is made for every 60.degree. phase angle. In other words, the circuit fails to detect a phase change occurring within the phase angle 60.degree..
The adder 14 produces an output signal with the frequency six times those of the three-phase AC signals S1U, S1V and S1W. Such an output signal is applied to the low-pass filter 18. Therefore, a proper phase detection is impossible for a phase variation arising at a rate higher than the frequency six times that of the three-phase AC signal.
Particularly, when the frequency of the three-phase AC signal is low, it is impossible to set an in-phase condition at a high speed.